From U.S. Pat. No. 3,471,253, it is known to obtain a waterglass solution by introducing 42% by weight sodium hydroxide solution and sand (silica) in a weight ratio of about 2:1 into a stirred autoclave and allowing the mixture to remain therein for 3 hours at 210.degree. C. and 16 bar. The hot sodium silicate solution taken out after cooling of the autoclave content to 85.degree. C. contains, after excess sand and other impurities have been filtered off, 57.5% of solids and has an SiO.sub.2 /Na.sub.2 O ratio of 1.64:1.
Crystalline, anhydrous sodium silicates having a layer structure and an SiO.sub.2 /Na.sub.2 O molar ratio of (1.9 to 3.5):1 are produced by the process according to German Offenlegungsschrift 3,718,350 by treating waterglass solutions having a solids content of 20 to 65% by weight in a spray-drying zone to form a water-containing amorphous sodium silicate, the exit gas flowing out of the spray-drying zone having a temperature of at least 140.degree. C. The water-containing amorphous sodium silicate is heat-treated in an ignition zone for 1 to 60 minutes at 500.degree. to 800.degree. C. in the presence of at least 10% by weight of recycle material, which was obtained by mechanical comminution of crystalline sodium silicate previously discharged from the ignition zone.
A disadvantage in the last mentioned process is that the material obtained in spray-drying takes up a large volume because of its low bulk density of 100 to 250 g/l and generates a lot of dust. Moreover, the use of recycle material during the heat treatment causes considerably greater expense on equipment and, because of the higher throughput of material, requires a rotary tube of greater dimensions. Finally, due to the use of recycle material at an SiO.sub.2 /Na.sub.2 O molar ratio of 2:1, a high proportion of the high-temperature modification of sodium disilicate (.alpha.--Na.sub.2 Si.sub.2 --O.sub.5) is formed but it is not the high-temperature modification which is desired, but the .sym.-modification because of its better builder properties.